After formatting each sector, the host must send
new values for C, H, R and N to the FDC for the
next sector on the track. The R value (sector
number) is the only value that must be changed
by the host after each sector is formatted. This
allows the disk to be formatted with
nonsequential sector addresses (interleaving).
This incrementing and formatting continues for
the whole track until the FDC encounters a pulse
on the IDX pin again and it terminates the
command.
Format A Track
The Format command allows an entire track to
be formatted. After a pulse from the IDX pin is
detected, the FDC starts writing data on the disk
including gaps, address marks, ID fields, and
data fields per the IBM System 34 or 3740
format (MFM or FM respectively). The particular
values that will be written to the gap and data
field are controlled by the values programmed
into N, SC, GPL, and D which are specified by
the host during the command phase. The data
field of the sector is filled with the data byte
specified by D. The ID field for each sector is
supplied by the host; that is, four data bytes per
sector are needed by the FDC for C, H, R, and
N (cylinder, head, sector number and sector size
respectively).
Table 28 contains typical values for gap fields
which are dependent upon the size of the sector
and the number of sectors on each track. Actual
values can vary due to drive electronics.
FORMAT FIELDS
SYSTEM 34 (DOUBLE DENSITY) FORMAT
GAP4a SYNC
GAP1 SYNC IDAM
C
Y
L
H
D
S
E
C
N
O
C
R
C
GAP2 SYNC
C
R
C
DATA
AM
80x
4E
12x
00
50x
4E
12x
00
22x
4E
12x
00
DATA
DATA
DATA
GAP3 GAP 4b
GAP3 GAP 4b
GAP3 GAP 4b
IAM
3x
C2
3x
3x FB
A1 F8
A1
FC
FE
SYSTEM 3740 (SINGLE DENSITY) FORMAT
GAP4a SYNC
GAP1 SYNC
C
Y
L
H
D
S
E
C
N
O
C
R
C
GAP2 SYNC
C
R
C
DATA
AM
40x
FF
6x
00
26x
FF
6x
00
11x
FF
6x
00
IAM
IDAM
FE
FB or
F8
FC
PERPENDICULAR FORMAT
GAP4a SYNC
GAP1 SYNC IDAM
C
Y
L
H
D
S
E
C
N
O
C
R
C
GAP2 SYNC
C
R
C
DATA
AM
80x
4E
12x
00
50x
4E
12x
00
41x
4E
12x
00
IAM
3x
C2
3x
A1
3x FB
A1 F8
FC
FE
60
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